Indirect thermal sensing system for a battery charger

ABSTRACT

A battery charger including a battery temperature monitoring device. The battery temperature monitoring device includes a first thermistor bonded to one of the terminals of the battery charger to provide a temperature signal of the temperature of the terminal, and a second thermistor positioned within the battery charger to provide a temperature signal of the ambient temperature. The temperature signals from both the first and second thermistors are applied to a temperature monitoring circuit that compares the temperature signals to a known discharge rate of an RC circuit. A microprocessor receives output signals from the temperature monitoring circuit and, using an algorithm, determines the actual temperature of the battery.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. Pat. application No. 08/834,375, filedApr. 16, 1997, now U.S. Pat. No. 5,874,825 issued Feb. 23 1999.

This application is related to United States Provisional ApplicationSer. No. 60/017,404, filed May 3, 1996.

FIELD OF THE INVENTION

The present invention relates to a device for monitoring batterytemperature during charging of an electrical battery or battery pack.More particularly, the present invention relates to a device formonitoring battery temperature during charging of a battery packincluding measuring a battery terminal temperature and a battery chargerambient temperature using an analog-to-digital conversion technique.

BACKGROUND OF THE INVENTION

It is known in the prior art to monitor a parameter associated with thecharging condition of battery packs in order to terminate charging assoon as a full level of charge is reached. This prevents battery packsfrom being damaged and thus prolongs their service life. Continuedcharging of the battery pack beyond the full charging level may haveserious disadvantages for all types of batteries, especially thenickel-cadmium batteries.

A number of different systems are known in the art to detect fullbattery charge in a charging system. One method of determining fullcharge is by monitoring the temperature of the battery pack. These typesof systems, however, suffer the drawbacks of repeated repetition of hightemperature, low charging efficiency, and problems with safety indefective cells. A second type of charging system uses a voltage cutofftechnique. These types of systems have proved to be unsatisfactory inthat temperature variations lead to large voltage variations, and thus,an inaccurate full charge determination. Another type of charging systemincorporates the termination of the charging as a function of the timeof charging. These types of systems have been unreliable in that it isdifficult to accurately tell what the state of the charge of the batterypack is at the initiation of the charging sequence

A more reliable method of charging has been disclosed in which thecharging device monitors the slope of the voltage-time curve for aparticular battery. Since the voltage-time charging curve for aparticular battery will always be substantially the same, it is possibleto determine different points on the curve which represent differentpoints in the charging sequence, and thus it is possible to determinewhich point of the curve represents full charge.

A quick charging system incorporating a type of slope monitoringtechnique is disclosed in U.S. Pat. Nos. 4,388,582 and 4,392,101, bothto Saar et al. The Saar et al. patents disclose a quick chargingtechnique which analyzes the charging of a battery by noting inflectionpoints which occur in the curve as the electrochemical potential withinthe battery changes with respect to time. By determining specificinflection points in the charging curve, it is possible to accuratelyterminate the rapid charging when the battery receives full charge.

The inflection point type analysis can be illustrated by viewing FIG. 1.FIG. 1 is a typical voltage-time curve of a nickel-cadmium ("Ni-Cad")battery. As is apparent, the voltage continuously rises as the chargingtime increases until it gets to a maximum charge point. Although thespecific values of the curve may differ from battery to battery, thegeneral shape of the curve is typical for all nickel-cadmium batteries.Further, every type of rechargeable battery will have a voltage-timecurve indicative of its type.

As is apparent, the curve can be separated into five distinct regions.Region I represents the beginning of the charging sequence. In thisregion, the voltage characteristics are somewhat unreliable and may varyfrom battery to battery in accordance with its prior history of beingcharged and discharged. It is for this reason that region I is shown asa dotted line. Further, this region is not important in the chargingsequence since it is generally traversed within a relatively shortperiod of time after the start of the charging sequence.

After approximately 30 to 60 seconds of starting the charging sequence,the charging curve will enter the more stable region of region II.Region II is generally the longest region of the charging sequence, andis marked by most of the internal chemical conversion within the batteryitself. As is apparent, the voltage of the battery does not increasesubstantially over this region. At the end of region II is an inflectionpoint A in the curve. Inflection point A represents a transition fromregion II to region III and is noted by a point where the slope of thecurve changes from a decreasing rate to an increasing rate.

Region III is the region in which the battery voltage increases quiterapidly. As the battery reaches its fully charged condition, theinternal pressure and temperature of the battery also increasesubstantially. When these effects begin to take over, the increase inbattery voltage begins to taper off. This is noted as the inflectionpoint B.

Region IV represents the fully charged region between inflection point Band the peak of the curve represented by point C. The voltage onlystabilizes at point C for a short period of time. If charging continues,the additional heating within the battery will cause the voltage of thebattery to decrease and, in addition, may damage the battery.

By analyzing the inflection points of the voltage-time curve, it can bedetermined at what point the battery has reached maximum charge This isdone by first determining inflection point A and then looking forinflection point B. Once inflection point B is observed, the chargingprocess can be discontinued. Since it is possible to determine theinflection points very readily and accurately, it is possible to haltthe charging process, or maintain the charging process at a maintenancecharge, following detection of the second inflection point.

A battery charging system which incorporates the above-describedanalysis of the voltage-time curve of a rechargeable battery isdescribed in U.S. Pat. No. 5,352,969, assigned to the assignee of thepresent invention and herein incorporated by reference. The systemdescribed in the U.S. Pat. No. 5,352,969 patent is capable of chargingbatteries of different voltages on the same charger. A typical batterycharger which can incorporate the charging system shown in the U.S. Pat.No. 5,352,969 patent is shown in U.S. Pat. No. 5,144,217. These types ofbattery chargers and others incorporating the voltage-time curveanalysis also incorporate a temperature monitoring system for theprotection of the batteries and the charging system in general. Thetemperature monitoring systems in general incorporate a temperaturesensitive element, such as a thermistor, which must be brought intoclose association with or in contact with the battery cell.

One prior art technique for monitoring battery cell temperature consistsof locating a thermistor during charging in a suitable recess in thebattery pack in a position adjacent to one of the battery cells. Thistechnique is somewhat inaccurate and thus unsatisfactory in practicesince although the thermistor is located adjacent a battery cell, it maynot be brought into contact with the battery cell. The thermistor,therefore, fails to detect the actual temperature of the battery cell,and instead detects the ambient temperature adjacent to the batterycell. The inaccurate battery cell temperature may lead to theovercharging of the battery or the battery pack.

Another version of these prior art battery charges requires the operatorto position the thermistor on the battery pack at the time of chargingthe battery. If the operator forgets to position the thermistor orpositions it incorrectly, the charging of the battery pack will continueuntil it is completely destroyed. Additional versions of these prior artchargers with temperature monitoring systems incorporate the thermistorin the structure of the battery pack during its manufacture. This notonly increases the complexity and costs associated with each batterypack, it also requires that a correct connection be established betweenthe battery charger and the thermistor when the battery pack is attachedto the charger. An incorrect connection of the thermistor with thecharger will lead to incorrect temperature information which will leadto the charging of the battery pack until it is destroyed. Moreover, inpractice, it is found that the electrical connections of the thermistoror the thermistor itself deteriorate when the battery pack is in use,particularly when the battery pack is used with portable tools thatvibrate.

An attempt has been made to obviate the disadvantages of these prior artbattery chargers which rely on a thermistor in the structure of thebattery pack by incorporating the thermistor as part of the charger andpositioning the thermistor on one of the terminals of the charger whichcommunicate with the battery pack being charged. A detection system canbe incorporated into the charger such that when connection is detectedbetween the thermistor and the rechargeable battery cell of the batterypack, the charging process is permitted. Otherwise, the charging processis prohibited.

While these later designs for battery chargers which include athermistor on one of the charger terminals have successfully resolvedsome of the problems associated with charging battery packs, there arestill some unresolved issues relating to these chargers. The positioningof the thermistor on the terminal of the charger only places thethermistor in relatively good thermally conductive relationship with thebattery cells. The chain of this conductivity is through the terminal,to the battery pack terminal and to the battery cells themselves. Thus,the temperature being read by the thermistor is not the temperature ofthe battery cells. It is the temperature of the battery charger terminalwhich is being heated by the battery cells producing a time delaybetween the actual temperature of the battery cells and the temperaturebeing sensed by the thermistor on the battery terminal.

Accordingly, what is needed is a technique for determining the actualtemperature of the battery cells being charged using the output of atemperature sensing device, preferably a thermistor, which is a part ofthe battery charger and is preferably located on one of the terminals ofthe charge. This would provide the complexity and cost savingsassociated with having the temperature sensing element a part of thebattery charger while simultaneously providing for an accuratedetermination of the temperature for the battery cells.

SUMMARY OF THE INVENTION

The present invention provides the art with a battery charger whichincludes a first temperature sensing element, preferably a thermistor,in intimate contact with one of the terminals of the battery charger. Asecond temperature sensing element, also preferably a thermistor, ispositioned within the battery charger to sense ambient temperature. Atemperature signal from both the first temperature sensing element andthe second temperature sensing element are applied to ananalog-to-digital conversion circuit. The analog-to-digital conversioncircuit uses a known discharge rate of an RC circuit to convert thenon-linear analog temperature signals into a linear digital equivalent.A microprocessor located within the charger receives output signals fromthe temperature monitoring circuit and, using an algorithm, approximatesthe actual temperature for the battery cells.

Other advantages and objects of the present invention will becomeapparent to those skilled in the art from the subsequent detaileddescription, appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which illustrate the best mode presently contemplatedfor carrying out the invention:

FIG. 1 is a graph showing battery voltage versus time for a chargingsequence of a nickel-cadmium battery;

FIG. 2 is a schematic diagram of a battery monitoring circuit includinga temperature monitoring system in accordance with the presentinvention;

FIG. 3 is a graph showing thermistor resistance versus temperature;

FIG. 4 is a graph showing analog-to-digital voltage versus temperature;and

FIG. 5 is a graph showing logarithmic analog-to-digital counts versustemperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the preferred embodiment directed to abattery monitor including a temperature monitoring system is merelyexemplary in nature and is in no way intended to limit the invention orits applications or uses.

U.S. Pat. No. 5,352,969 mentioned above, and incorporated herein byreference, discloses a logarithmic analog-to-digital conversiontechnique to convert the analog battery voltage signal to a digitalsignal to be processed by a microprocessor. The range of rated voltagesof battery packs used in different power tools varies depending on thenumber of cells in the pack. The actual voltage exhibited by differentbattery packs can vary beyond the rated voltage, for example, from lessthan one volt per cell to greater than two volts per cell. In order toprovide adequate charge analysis resolution for all rated and actualvoltages within this range during charging, the '969 patent uses atechnique for automatically scaling the analog battery voltage signal tobe within a specific range, regardless of the rated output voltage ofthe battery pack, in conjunction with the logarithmic analog-to-digitalconversion technique. To accomplish this, the battery chargerincorporates a prescaler circuit comprising a variable voltageattenuator circuit that is selectably settable by the microprocessor bya plurality of analog switching devices. Since the analog-to-digitalconversion circuitry uses a constant reference voltage, such as five (5)volts, it is necessary to attenuate the battery voltage signal suppliedto the analog-to-digital conversion circuitry to be less than thereference voltage. The prescaler circuit attenuates the voltage from thebattery pack, and is selectively variable by the switching device to setthe attenuated voltage to be at a steep part of a discharge curve of theanalog-to-digital conversion circuit to increase accuracy. Thisattenuated voltage is about 3.3 volts in one example. Accordingly,regardless of the rated output voltage of the particular battery packbeing charged, full charge will not exceed the reference voltage.Because modern battery charges are microprocessor controlled, it isnecessary to convert the analog battery voltage signal to a digitalsignal to monitor the charging sequence to determine, for example, theinflection points in the charging curve of FIG. 1. The present inventionproposes a much simplified analog-to-digital conversion circuitry tothat disclosed in the '969 patent. However, the basic operation of theanalog-to-digital converter circuit in the '969 patent is effectivelythe same as that discussed below, and thus the operation of thatconversion circuit is applicable to this discussion.

FIG. 2 shows a battery monitoring circuit 10 that monitors the voltageand temperature of a battery pack 12 to be charged. The battery pack 12includes a certain number of cells depending on the particular pack andcan have various rated voltages. The circuit 10 is microprocessorcontrolled by a microprocessor 14. The actual charging circuitry is notshown in this figure, however, as will be understood by those skilled inthe art, can be any suitable battery charging circuitry, such asdiscussed in the '969 patent, for the purposes of the present invention.The battery monitoring circuit 10 includes an analog-to-digitalconversion circuit 16 that includes a prescaler 18 acting as a voltagedivider network made up of resistors R₁, R₂, R₃, and R₄. The prescaler18 attenuates the analog battery voltage signal depending on the valueof the resistors R₁, R₂, R₃ and R₄ and which resistors are in theprescaler 18. The attenuated battery voltage signal from the batterypack 12 is applied to a positive terminal of a comparator 20. Theresistors R₁, R₂, R₃, and R₄ are switched into and out of the prescaler18 by a series of switches 22 internal to the microprocessor 14. When aswitch 22 connected to a particular resistor is open, that resistor isconnected to high impedance, and thus is taken out of the prescaler 18.If the particular switch 22 is closed, the resistor is connected toground, and acts to help attenuate the battery voltage signal from thebattery pack 12. To determine if a battery has in fact been placed inthe charger, the microprocessor 14 closes all of the switches 22 formaximum attenuation.

The negative terminal of the comparator 20 is connected to a node 26between a resistor R_(LogA/D) and a capacitor C_(LogA/D) of an RCcircuit 28. An opposite terminal of the resistor R_(LogA/D) from thenode 26 is connected to an output pin of the microprocessor 14 which isinternally connected to a switch 30. The switch 30 is connected to asuitable voltage potential, here five volts, or ground. When the switch30 is switched to the plus five volt terminal, the capacitor C_(LogA/D)is charged through the resistor R_(LogA/D) until it is fully charged.Once the capacitor C_(LogA/D) is fully charged, a voltage reading of thebattery pack 12 can be taken by switching the switch 30 to the groundterminal. When the switch 30 is switched to the ground terminal, thecapacitor C_(LogA/D) begins to discharge at a rate depending on thevalue of the resistor R_(LogA/D) and the capacitor C_(LogA/D). At thesame time as when the switch 30 is switched to ground, themicroprocessor 14 starts a counter to count known clock pulses. When thecapacitor C_(LogA/D) has discharged to a voltage level that is less thanthe attenuated voltage level of the battery pack 12 applied to thepositive terminal of the comparator 20, the comparator 20 will switchhigh causing a plus five volt potential to be applied to an input pin ofthe microprocessor 14 through a pull-up resistor R₅. When this happens,the microprocessor 14 stops counting the clock pulses, and stores thecounted clock pulse value. This stored digital clock pulse count is thedigital conversion of the analog voltage signal of the battery pack 12as attenuated by the prescaler 18. Thus, depending on the configurationof the switches 22 establishing the attenuation of the prescaler 18, themicroprocessor 14 can calculate the voltage of the battery pack 12 basedon the discharge time of the capacitor C_(LogA/D), and the values of thecapacitor C_(LogA/D) and the resistor R_(LogA/D). A more detaileddiscussion of the operation of the analog-to-digital conversion circuit16 can be found in the '969 patent.

The attenuation of the battery voltage signal from the battery pack 12as established by the switches 22 is set to be approximately 3.3 voltsso that the attenuated voltage measurements of the battery pack 12 aretaken at an area of the discharge curve of the capacitor C_(LogA/D)suitable for an increased signal to noise ratio, and is within themaximum voltage potential of the system. In other words, the attenuatedvoltage from the battery pack 12 cannot exceed the reference voltage orthe comparator 20 will not indicate a high signal at the appropriatetime. It is preferred that the attenuated voltage be at the steeper partof the discharge curve where the voltage of the capacitor C_(LogA/D) ischanging more rapidly for increased accuracy.

The battery monitor circuit 10 also includes a temperature check circuit34 according to the present invention. The temperature check circuit 34includes a thermistor 36 that acts as a temperature sensor to measureambient temperature within the battery charger, and a thermistor 38 thatacts as a temperature sensor to measure terminal temperature of abattery charger terminal. In the embodiment being discussed herein, thethermistor 38 is physically bonded to the negative terminal of thebattery charger. However, in an alternate embodiment, the thermistor 38can be attached to either the positive or negative terminal of thebattery charger. As is well understood in the art, thermistors aretemperature sensitive devices whose resistance changes with respect totemperature. Other types of temperature sensing devices suitable fordetermining the ambient temperature of the battery charger and thetemperature of the battery charger terminal can be used in place of thethermistors 36 and 38 within the scope of the present invention. FIG. 3is a graph of temperature on the horizontal axis and resistance on thevertical axis to show the relationship of the resistance of a thermistorto temperature. As is apparent from this graph, this relationship is notlinear.

A resistor 40 is provided in series with the thermistor 36, and aresistor 42 is provided in series with the thermistor 38. Thecombination of the resistor 40 and the thermistor 36 makes up onevoltage divider network, and the combination of the resistor 42 and thethermistor 38 make up another voltage divider network. The voltagedivider networks divide a five volt potential applied to a terminal ofthe resistors 40 and 42 opposite to the thermistor 36 and 38,respectively. Both the resistors 40 and 42 are 82 k ohm resistors forthe purposes of the present invention. However, as will be appreciatedby those skilled in the art, this value may vary from battery charger tobattery charger. The voltage at a node 44 between the resistor 40 andthe thermistor 36, and the voltage at a node 46 between the resistor 42and the thermistor 38 is a voltage divided analog temperature signal tobe converted to a digital signal. The voltages at the nodes 44 and 46change as the temperature of the thermistors 36 and 38 changes. Thisrelationship with respect to temperature is given in the graphrepresented in FIG. 4, where the A/D voltage is given on the verticalaxis and the temperature is given on the horizontal axis.

The voltage signal at the node 44 is applied to a positive terminal of afirst comparator 48. Likewise, the voltage signal at the node 46 isapplied to a positive terminal of a second comparator 50. An output ofthe first comparator 48 is applied to an ambient sensor input of themicroprocessor 14, and an output of the second comparator 50 is appliedto a charger terminal input of the microprocessor 14. The combination ofthe thermistor 36, the resistor 40 and the first comparator 48 makes upan ambient temperature sensing portion of the temperature check circuit34, and the combination of the thermistor 38, the resistor 42, and thesecond comparator 50 makes up a charger terminal sensing portion of thetemperature check circuit 34.

To determine the ambient and charger terminal temperatures, themicroprocessor 14 applies a five volt potential to the RC circuit 28through the switch 30. The capacitor C_(LogA/D) in the RC circuit 28 ischarged through the series resistor R_(LogA/D) by the five voltpotential. Once the capacitor C_(LogA/D) is charged, the five voltpotential from the microprocessor 14 is applied to the negative terminalof both the comparators 48 and 50 through node 26. As the ambienttemperature varies, the resistance of the thermistor 36 will vary, thusaltering the potential applied to the positive terminal of thecomparator 48. For example, as ambient temperature goes up, theresistance of the thermistor 36 goes down, increasing the voltagepotential applied to the positive terminal of the comparator 48.Likewise, as the ambient temperature goes down, the resistance of thethermistor 36 goes up, reducing the voltage potential applied to thepositive terminal of the comparator 48. Therefore, the potential appliedto the positive terminal of the comparator 48 is determined by theambient temperature in the battery charger. The voltage potentialapplied to the positive terminal of the comparator 50 changes in thesame way with respect to changes in the battery terminal temperature assensed by the thermistor 38.

When the switch 30 is closed and is applying the five volt potential tothe RC circuit 28 and the capacitor C_(LogA/D) is charged, the potentialon the negative terminals of the comparators 48 and 50 will always begreater than the potential on the positive terminals of the comparators48 and 50 because of the voltage divider networks. Therefore, the outputof the comparators 48 and 50 will be low, and thus a low signal will beapplied to the microprocessor 14 at the ambient sensor input and thebattery charger terminal input. When the microprocessor 14 is instructedto take a temperature reading, it will switch the switch 30 to groundeliminating the five volt potential applied to the capacitor C_(LogA/D).When a ground potential is applied to the RC circuit 28, the capacitorC_(LogA/D) will begin to discharge through the resistor R_(LogA/D), andthe voltage potential applied to the negative terminals of thecomparators 48 and 50 will begin to decrease. Eventually, the potentialapplied to the negative terminal of the comparators 48 and 50 will fallbelow the potential applied to the positive terminals of the comparators48 and 50 as determined by the resistances of the thermistors 36 and 38and the discharge characteristics of the capacitor C_(LogA/D). When thisoccurs, the output of the comparators 48 and 50 will become highimpedance. Of course, unless the ambient temperature and the chargerterminal temperature are the same, the output of the comparators 48 and50 will not go high impedance at the same time. When the output of thecomparators 48 and 50 are high impedance, a five volt potential appliedto the ambient sensor input and the charger terminal sensor input to themicroprocessor 14 as applied through 100 k resistors 52 and 54 willcause the inputs to go high.

When the microprocessor 14 switches the switch 30 to the groundpotential, an internal counter within the microprocessor 14 beginsincrementing at a known clock frequency to time the discharge of thecapacitor C_(LogA/D). A separate internal counter can be used for boththe ambient temperature and the battery terminal temperature. When thecapacitor C_(LogA/D) discharges to a level that causes the output of thecomparators 48 and 50 to go high impedance, as determined separately atthe ambient sensor input and the charger terminal sensor input, themicroprocessor 14 will stop counting and use the accumulated countvalues to determine the temperatures of the thermistors 36 and 38, andthus the ambient temperature and the charger terminal temperature. Sincethe discharge time constant of the RC circuit 28 is known, and the clockrate of the microprocessor's clock is known, a predeterminedrelationship exists between the number of accumulated counts and thedischarge level of the capacitor C_(LogA/D). In one embodiment, theaccumulated count value is applied to a preset look-up table in themicroprocessor 14 to give the temperature values.

Because the A/D voltage potential applied to the positive terminal ofthe comparators 48 and 50 is not linear with respect to temperature, aproportional number of counts does not give a proportional change intemperature throughout the range of temperatures sensed by thethermistors 36 and 38. In other words, fifty counts may represent atemperature of 50° C., but one hundred counts will not represent 100° C.By applying the non-linear A/D voltage potential to the positiveterminals of the comparators 48 and 50 and the discharge voltage of theRC circuit 28 to the negative terminals of the comparators 48 and 50,the counts generated by the microprocessor 14 during the discharge ofthe RC circuit 28 provides a linearization of the analog signalrepresenting the temperatures to digital counts. This relationship isshown in FIG. 5 as temperature on the horizontal axis and log A/D countson the vertical axis.

In one embodiment, the practical temperature range of the ambientbattery charger temperature and the battery charger terminal temperatureis between 13 and 105° C. Using an 8 bit counter in the microprocessor14 generates 0-255 counts for this range of temperature. Therefore, theresolution of the temperature can be given as 255 counts divided by thenumber of degrees in the range.

According to one embodiment of the invention, the temperature of thebattery pack 12 is determined in the following manner. The chargerterminal temperature is monitored before the battery pack 12 is insertedinto the battery charger, and its value is represented as T₀. After thebattery pack 12 is inserted into the battery charger, subsequent chargerterminal temperature readings with the battery pack 12 in contact withthe terminal are represented as T₁. The ambient temperature within thebattery charger is also monitored during the charging process, and thisvalue is represented as T_(A). After an elapse of a suitable time periodfollowing the insertion of the battery pack 12 into the charger to allowstabilization of the thermal system, for example one minute, themicroprocessor 14 calculates T_(BATT) to determine the temperature ofthe cells in the battery pack 12 by the following equation.

    T.sub.BATT =C.sub.1 T.sub.1 +C.sub.2 T.sub.0 +C.sub.3 T.sub.A +C.sub.4

The coefficient C₁, C₂, C₃ and C₄ are selected to provide the mostaccurate response for the battery cell temperatures in a particularsituation. These coefficients can be derived by experimental proceduresand a linear curve fit mathematical technique. For a particularembodiment disclosed herein, C₁ =2.25 C₂ =-0.625, C₃ =-0.5 and C₄ =-4.

The equation above provides a suitable transfer function to provide amodel to accurately estimate battery temperature. With the temperatureof the battery pack 12 known, the battery charger can direct thecharging process while monitoring battery temperature. For example, thebattery charger may include a cool-down period for excessively highbattery packs and/or adaptive charge termination schemes based onbattery pack temperatures at the beginning of the charge cycle. Themonitoring circuit 10 will continue monitoring temperature until thebattery pack 12 does cool down. In addition, the controller can ensurethat critical temperature levels are not exceeded during the chargingprocess.

As discussed above, the temperature check circuit 34 uses a thermistor,the thermistor 36, bonded to the negative battery contact to determinebattery contact temperature. The thermistor 36 is connected to thetemperature check circuit 34 by a fine gauge wire. If the wire to thethermistor 36 were to break, the overtemperature system of the circuit34 would be disabled, and the system would default to a battery coolposition even if the battery was hot, possibly too hot to be charged.Thus, it is desirable to provide some type of fail-safe design thatenables a broken thermistor wire to be detected. According to one aspectof the present invention, such a system is provided, where the systemdefaults to a "broken wire" condition if no indication of an "intact"wire is reached within a predetermined time of the beginning ofcharging. The system then suspends charging current and enters a"problem mode" for example, a blinking LED. In a preferred example, thispredetermined time interval is twenty minutes. Of course, twenty minutesis used in a preferred embodiment, but as will be appreciated by thoseskilled in the art, other time limits will also be applicable within thescope of the invention.

The thermistors 36 and 38 are limited in temperature sensing range, inone embodiment from 13° C. (55° F.) to 105° C. (221° F.). If thethermistor wire breaks, then for the next charging sequence, thethermistor temperature reading locks to 13° C. However, the system doesnot know if the thermistor wire has broken, or the negative terminal isactually less than 13° C. If a hot battery is placed in a charger, andis not allowed to properly cool prior to charging, degrade performanceof battery could exist by continually recharging a hot battery. Thus, ifthe thermistor wire is broken, the possibility exists that a hot batteryinserted into the charger would not be detected because the terminaltemperature thermistor would read cool.

The present invention proposes determining if the thermistor wire isbroken by the existence of three conditions. These conditions are theterminal contact temperature is continually maintained at the minimaltemperature, (13° C. in the preferred embodiment), the ambienttemperature, as measured by the thermistor 36, is greater than someminimal value, such as 20° C. in a preferred embodiment, and the chargecurrent has been charging the battery for at least some minimal value,for example, twenty minutes in one embodiment. If all three of theseconditions exist, the microprocessor 14 determines that the contactthermistor wire is broken and enters the problem mode.

Software is provided in the microprocessor 14 to determine whether thecontact thermistor wire is broken. This software performs the brokenwire test on every new battery pack insertion and charging sequence, anda broken wire flag is set in the software when the three conditionsexist. The determination of the contact terminal temperature iscontinuously evaluated once every second on a specific example. If thistemperature measurement is ever not equal to the minimal 13° C. reading,then the software indicates that the wire is not broken. Also, thesoftware determines the charging time to determine when it has reachedthe minimum time of twenty minutes. And, the ambient temperature ismonitored.

The temperature check circuit 34, as described above, is discussed incombination with a particular battery charger suitable for chargingvarious batteries having a number of battery cells. However, it isstressed that the temperature check circuit 34 has a wider range ofapplications for various other battery chargers having different numbersof cells, and is not particularly limited to the quick-charge batterytype charger.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A temperature monitoring device for determiningthe temperature of a battery pack being charged in a charging device,said charging device including a charging terminal, said monitoringdevice comprising:an ambient temperature sensor positioned to senseambient temperature within the battery charger and providing atemperature signal of the sensed ambient temperature; a terminaltemperature sensor positioned to sense the temperature of the chargingterminal and providing a temperature signal indicative of thetemperature of the terminal; a temperature monitoring circuit responsiveto the temperature signals from the ambient temperature sensor and theterminal temperature sensor, said temperature monitoring circuitproviding output signals indicative of the temperature signals; andcontroller means responsive to the output signals from the monitoringcircuit for determining the temperature of the battery pack based on thetemperature signals in accordance with the equation:

    T.sub.BATT =C.sub.1 T.sub.1 +C.sub.2 T.sub.0 +C.sub.3 T.sub.A +C.sub.4

where, T_(BATT) is the temperature of the battery pack; T₀ is thetemperature of the charging terminal before the battery pack is insertedinto the charging device; T₁ is the temperature of the charging terminalafter the battery pack is inserted into the charging device; and T_(A)is the ambient temperature within the battery charger.
 2. A batterycharger for charging a battery, said battery charger comprising:aterminal for providing electrical energy to the battery; first sensingmeans for sensing the temperature of the terminal; second sensing meansfor sensing ambient temperature in the vicinity of the battery charger;controller means for controlling the battery charger in response to thetemperature of the battery which is determined by the controller meansin accordance with the equation:

    T.sub.BATT =C.sub.1 T.sub.1 +C.sub.2 T.sub.0 +C.sub.3 T.sub.A +C.sub.4

where, T_(BATT) is the temperature of the battery; T₀ is the temperatureof the charging terminal before the battery is inserted into thecharging device; T₁ is the temperature of the charging terminal afterthe battery is inserted into the charging device; and T_(A) is theambient temperature within the battery charger.